1. Field of the Invention
The present invention relates to a process for manufacturing a cable.
In particular, the present invention relates to a process for manufacturing an electrical cable for transmission or distribution of electrical power at medium or high voltage.
More in particular, the present invention relates to a process for manufacturing an electrical cable having a structure with a very compact design.
2. Description of the Related Art
In the present description, the term medium voltage is used to refer to a tension typically from about 10 to about 60 kV and the term high voltage refers to a tension above 60 kV. Generally, the term low voltage refers to a tension lower than 10 kV, typically greater than 100 V. The term very high voltage is also sometimes used in the art to define voltages greater than about 150 or 220 kV, up to 500 kV or more.
Cables for power transmission or distribution at medium or high voltage generally have a metal conductor which is surrounded, respectively, by an inner semiconductive layer, an insulating layer and an outer semiconductive layer. In the following of the present description, said predetermined sequence of elements will be indicated with the term of “cable core”.
In a position radially external to said core, the cable is provided with a metal shield (or screen), usually made of aluminium, lead or copper. Generally, the metal shield consists of a continuous tube or of a metallic tape shaped according to a tubular form and welded or sealed to ensure hermeticity. Alternatively, the metal shield is formed of a plurality of metal wires.
The metal shield performs an electrical function by creating, inside the cable, as a result of direct contact between the metal shield and the outer semiconductive layer of the cable core, a uniform electrical field of the radial type, at the same time cancelling the external electrical field of said cable.
The metal shield may also provide hermeticity against the exterior of the cable by interposing a barrier to water penetration in the radial direction.
A further function of the metal shield is that of withstanding short-circuit currents.
In a configuration of the unipolar type, the cable is provided with a polymeric oversheath in a position radially external to the metal shield mentioned above.
Moreover, cables for power transmission or distribution are generally provided with one or more layers for protecting said cables from accidental impacts which may occur on their external surface.
Accidental impacts on a cable may occur, for example, during transport thereof or during the laying step of the cable in a trench dug into the soil. Said accidental impacts may cause a series of structural damages to the cable, including deformation of the insulating layer and detachment of the insulating layer from the semiconductive layers, damages which may cause variations in the electrical voltage stress of the insulating layer with a consequent decrease in the insulating capacity of said layer.
In the cables which are currently available in the market, for example in those for low or medium voltage power transmission or distribution, metal armours capable of withstanding said impacts are usually provided in order to protect said cables from possible damages caused by accidental impacts. Generally, said armours are in the form of tapes or wires (preferably made of steel), or alternatively in the form of metal sheaths (preferably made of lead or aluminum). An example of such a cable structure is described in U.S. Pat. No. 5,153,381.
European Patent No 981,821 in the name of the Applicant, discloses a cable which is provided with a layer of expanded polymeric material in order to confer to said cable a high resistance to accidental impacts, said layer of expanded polymeric material being preferably applied radially external to the cable core. Said proposed technical solution avoids the use of traditional metal armours, thereby reducing the cable weight as well as making the production process thereof easier.
The Applicant has perceived the need of providing a cable with a compact cable core, i.e. with a cable core design having reduced thicknesses of the semiconductive layers and of the insulating layer with respect to conventional cables, in order to reduce the cable size and weight for advantageously enhancing the handling, flexibility and transport thereof, without decreasing the overall electrical and mechanical resistance properties of the cable.
However, the Applicant has noted that the production of such a compact cable core can not be carried out—at the desired manufacturing speed—by using the manufacturing processes known in the art which are unsuitable for providing the desired results.
In order to produce a compact cable core which is provided with a very thin inner semiconductive layer (i.e. having a thickness lower than or equal to 0.4 mm), the Applicant has noted that the known extrusion techniques, according to which the flows of the different materials forming the cable core constitutive layers are kept separate from each other and separately extruded onto the cable core being formed, give rise to a plurality of drawbacks which do not allow the desired cable core to be produced at a reasonable speed.
For instance, in case a remarkable reduction of the thickness of the inner semiconductive layer is desired to be obtained, the known cable manufacturing processes give rise to the formation of a non-homogeneous thickness of the inner semiconductive layer, either in the longitudinal or in the radial directions, as well as tearings thereof during extrusion of the inner semiconductive layer onto the cable conductor. This is due to the fact that, while moving along the extruder head, the conductor exerts a pulling force on the very thin extruded inner semiconductive layer, thereby causing the above mentioned defects to occur. This aspect is even more stressed when the cable conductor is moved along the extruder head at a predetermined feeding speed which is sufficiently high (e.g. at a conventional feeding speed of about 30 m/min) to allow an industrial productivity to be carried out. Therefore, the combination of a relatively high cable conductor feeding speed with a very thin inner semiconductive layer to be extruded generally produces a defective cable core which is unacceptable and thus discarded.
Moreover, in accordance with the known cable manufacturing processes, in case a reduced thickness of the inner semiconductive layer is requested to be obtained, the length of the extrusion channel—which is used for extruding the inner semiconductive layer—is sensibly greater than the average height thereof (the height of the channel is measured in a plane perpendicular to the channel longitudinal walls). This aspect causes a remarkable increase of the extrusion pressure inside the extruder head that is due to a decrease of the extrusion channel cross-section and, as a consequence, to an increase of the extruded material speed moving along the extrusion channel. Therefore, in order to reduce the pressure at the extruder head, the extrusion output of the inner semiconductive layer is set to a lower value so as to reduce the speed of the inner semiconductive in the extrusion channel, thereby negatively affecting the cable manufacturing process productivity.
Furthermore, in accordance with the known cable manufacturing processes, in case a reduced thickness of the inner semiconductive layer is requested to be obtained, a precise production and/or assembling of the dies which form the inner semiconductive layer extrusion channel remarkably influences the stability of the extruded material flux. As a consequence, a non-homogeneous distribution of the extruded material and a non-homogeneous thickness of the inner semiconductive layer onto the cable conductor can occur.
Conventional cable manufacturing processes are also known according to which a cable multilayer element is co-extruded onto the cable conductor by causing the single layers of said multilayer element to contact each other before being extruded onto the conductor so that the multilayer element is formed at a position which is upstream of the contacting point between the cable conductor and the cable multilayer element.
For instance, document U.S. Pat. No. 3,737,490 discloses a method of manufacturing an extruded composite covering of an electric cable on a continuously advancing core by means of a float-down process, said covering comprising two or more layers of different covering materials. The method comprises causing the core to pass through the core tube of an extrusion machine which feeds extruded, peripherally continuous layers of the covering materials simultaneously towards the outlet end of the extrusion machine; causing the extruded layers to come into complete and intimate interfacial contact upstream of the outlet end of the extrusion machine; effecting continuous treatment of the composite covering so formed by passing the covered core through a chamber hermetically sealed to the outlet end of the extrusion machine and containing a fluid medium at a super-atmospheric pressure; and, at the same time, injecting fluid under pressure into the interior of the core tube and maintaining the fluid at a pressure which is less than that of the fluid medium by an amount such that the pressure difference across the extruded composite covering at the extrusion orifice is sufficient to cause the extruded composite covering to collapse firmly on to the core as it emerges from the extrusion machine but is insufficient to force the extruded composite covering back along the core tube. Moreover, said document discloses a cross-head of an extrusion machine having at its outlet end an annular extrusion orifice defined by an outer die and an inner die which is secured to the forward end of a core tube extending through the head. Upstream of the extrusion orifice is an intermediate die. Semiconductive polyethylene in a plastic state is fed to the annular space between the inner die and the intermediate die through a supply passage and polyethylene in a plastic state is fed to the annular space between the intermediate die and the outer die through a supply passage. The intermediate die is so positioned with respect to the outer die and inner die that the extruded layers of the semi-conductive polyethylene and insulating polyethylene come into complete and intimate interfacial contact upstream of the extrusion orifice. By this method a composite covering comprising an inner semiconductive cross-linkable polyethylene layer of radial thickness 0.5 mm and an outer insulating cross-linkable polyethylene layer of radial thickness 2.8 mm can be applied to a sector-shaped conductor.
Document U.S. Pat. No. 4,093,414 discloses a die by which thermoplastic insulating compounds can be co-extruded for applying a foam/skin insulation over a cable conductor, especially in the manufacturing process of a telephone wire. According to said document only one tip and one extrusion die are used for applying the two layers of insulating material (a first cellular insulating layer and a second solid insulating layer over said cellular layer) with a melt-flow separator between the supplies of insulation as they approach the end of the tip through which the conductor passes. The melt flow separator keeps the insulating materials from merging before they are close to the discharge end of the single tip, said melt flow separator terminating some distance back from the end of the tip so that the disruption of the cellular structure of the inner layer can be avoided.
Document EP-534,208 discloses an extrusion head for co-extruding at least two different plastic materials that are provided by means of two feed channels which open out into a common outlet die and into a slit-shaped homogenisation zone which serves to homogenise the stream of material. The homogenisation zone for the interior material extends essentially in the axial direction, whereas the homogenisation zone for the exterior material extends essentially in the radial direction. An elongate article can be sheathed by said at least two different plastic materials.